fix(library): survive a music disk that is not mounted

On the test server the host music folder was not mounted, so every stream and
fingerprint failed with no such file, and a scan of the empty folder would have
marked the whole library missing and purged it after 30 days.

- a scan of an empty source folder with known files aborts with a clear error
- fingerprinting skips such a source with one warning instead of one per file
- LIBRARY_HOST mounts use rslave, so a disk mounted after start shows up
- update.sh refuses to deploy with an empty LIBRARY_HOST

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

services/api/internal/library/fingerprint.go

@@ -198,6 +198,7 @@ func (fp *Fingerprinter) chromaprint() bool {
 func (fp *Fingerprinter) Run(ctx context.Context) {
 	withFP := fp.chromaprint()
 	roots := map[uuid.UUID]string{}
+	unavailable := map[uuid.UUID]bool{} // каталог источника пуст или не открывается — диск не смонтирован
 	var after uuid.UUID
 	for ctx.Err() == nil {
 		todo, err := fp.store.FilesToFingerprint(ctx, withFP, after, 200)
@@ -213,6 +214,9 @@ func (fp *Fingerprinter) Run(ctx context.Context) {
 				return
 			}
 			after = f.ID
+			if unavailable[f.SourceID] {
+				continue
+			}
 			root, ok := roots[f.SourceID]
 			if !ok {
 				if root, err = fp.rootOf(ctx, f.SourceID); err != nil {
@@ -220,12 +224,31 @@ func (fp *Fingerprinter) Run(ctx context.Context) {
 					continue
 				}
 				roots[f.SourceID] = root
+				if empty, err := dirEmpty(root); empty || err != nil {
+					fp.log.Warn("fingerprints: source folder is empty or unreadable, skipped — is the disk mounted?",
+						"source_id", f.SourceID, "root", root, "err", err)
+					unavailable[f.SourceID] = true
+					continue
+				}
 			}
 			fp.process(ctx, f, filepath.Join(root, filepath.FromSlash(f.RelPath)), withFP)
 		}
 	}
 }
 
+func dirEmpty(dir string) (bool, error) {
+	d, err := os.Open(dir)
+	if err != nil {
+		return false, err
+	}
+	defer func() { _ = d.Close() }()
+	names, err := d.Readdirnames(1)
+	if errors.Is(err, io.EOF) {
+		return true, nil
+	}
+	return len(names) == 0, err
+}
+
 func (fp *Fingerprinter) process(ctx context.Context, f FingerprintTodo, abs string, withFP bool) {
 	sum := f.SHA256
 	if sum == nil {